Method and apparatus for surface and ambient temperature estimation for portable devices

ABSTRACT

The surface temperature of a portable device is estimated. The portable device includes a sensor for detecting the internal temperature of the portable device. The portable device also includes circuitry for estimating the surface temperature, using the internal temperature and an ambient temperature of the portable device as input to a circuit model. The circuit model describes thermal behaviors of the portable device. The circuitry is operative to identify a scenario in which the portable device operates, and determine the ambient temperature using the scenario and at least the internal temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 15/859,518 filed on Dec. 30, 2017, and claims thebenefit of U.S. Provisional Application No. 62/463,005 filed on Feb. 24,2017, the entirety of which is incorporated by reference herein.

TECHNICAL FIELD

Embodiments of the invention relate to thermal management of portabledevices.

BACKGROUND

Modern portable devices, such as laptops, tablets, smartphones, smartwatches, gaming devices and other consumer electronics, are designed torun system and user space applications when the devices are held by orin contact with users. Thermal management for these devices regulatesnot only the internal temperature of the device to ensure safeoperation, but also the surface temperature of the device such that itis comfortable for the user to touch the device. Thermal managementprevents the device from generating excessive heat, as excesstemperature can degrade the performance, life, and reliability of thedevice. One form of dynamic thermal management is thermal throttling.Thermal throttling reduces the processor performance when the processorsin the device are heavily loaded, to thereby limit the internaltemperature and the surface temperature to acceptable levels.

One approach to estimating the surface temperature of a portable deviceis to use the internal temperature minus a fixed offset to calculate thesurface temperature. However, the difference (i.e., offset) between theinternal temperature and the surface temperature can vary from onedevice product to another, and can vary in the same device underdifferent usage scenarios and different levels of power consumption.Using such an oversimplified approach often leads to inaccurateestimation of the surface temperature, and may cause performance to bethrottled before the surface temperature reaches a predetermined limit.

Another approach to estimating the surface temperature of a portabledevice is to place a temperature sensor at the device surface; e.g., thedisplay. However, such a sensor detects the temperature of a limitedsurface area on the device. In some usage scenarios there may bemultiple heat sources located in different parts of the device, causingnon-uniform temperature rise on the device surface. Furthermore, indifferent usage scenarios there may be different heat sources inside thedevice, causing the surface temperature to rise on different parts ofthe device surface. Thus, a temperature sensor at a fixed surfacelocation may be unable to detect surface temperature change across theseusage scenarios.

Thus, there is a need for improvement in surface temperaturedetermination to thereby enable accurate dynamic thermal management of aportable device.

SUMMARY

In one embodiment, a method is provided for estimating a surfacetemperature of a portable device. The method comprises detecting aninternal temperature of the portable device; identifying a scenario inwhich the portable device operates; determining an ambient temperatureof the portable device using the scenario and at least the internaltemperature; an estimating the surface temperature using the internaltemperature and the ambient temperature as input to a circuit model thatdescribes thermal behaviors of the portable device.

In another embodiment, a portable device is provided to estimate asurface temperature of the portable device. The portable devicecomprises a sensor to detect an internal temperature of the portabledevice. The portable device further comprises circuitry to estimate thesurface temperature, using the internal temperature and an ambienttemperature of the portable device as input to a circuit model thatdescribes thermal behaviors of the portable device. The circuitry isfurther operative to identify a scenario in which the portable deviceoperates; and determine the ambient temperature using the scenario andat least the internal temperature.

The embodiments of the invention enable a portable device to estimateits surface temperature without the use of a surface temperature sensor.Estimation of the surface temperature allows the portable device toimprove its dynamic thermal management such that thermal throttling maybe performed when the surface temperature reaches a predetermined limit.Advantages of the embodiments will be explained in detail in thefollowing descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that differentreferences to “an” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean at leastone. Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is submitted that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

FIG. 1 illustrates an example of a device performing surface temperatureestimation according to one embodiment.

FIG. 2 illustrates a side view of a device that includes a sensor fordetecting the internal temperature according to one embodiment

FIG. 3A and FIG. 3B illustrate temperature estimators according to twoalternative embodiments.

FIG. 4A, FIG. 4B and FIG. 4C illustrate circuit models for estimatingthe surface temperature of a device according to alternativeembodiments.

FIG. 5 is a flow diagram illustrating a thermal management methodaccording to one embodiment.

FIG. 6A, FIG. 6B and FIG. 6C are flow diagrams illustrating methods forestimating the ambient temperature according to alternative embodiments.

FIG. 7 is a flow diagram illustrating a method for estimating thesurface temperature of a portable device according to one embodiment.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. In other instances, well-knowncircuits, structures and techniques have not been shown in detail inorder not to obscure the understanding of this description. It will beappreciated, however, by one skilled in the art, that the invention maybe practiced without such specific details. Those of ordinary skill inthe art, with the included descriptions, will be able to implementappropriate functionality without undue experimentation.

The surface temperature of a portable device is estimated according toembodiments of the invention. The surface temperature, also referred toas the skin temperature, is estimated based on the ambient temperatureand the internal temperature of the device. The estimation of thesurface temperature may be performed using a circuit model thatdescribes thermal behaviors of the device with a plurality of thermalcapacitors and thermal resistors. The use of the circuit model enablesthe estimation to be made with high accuracy, thus minimizingun-utilized thermal headroom of the device and premature thermalthrottling. In one embodiment, the ambient temperature may be estimatedbased on the internal temperature or a time-series of internaltemperatures (i.e., successive internal temperatures detected over atime period) of the device. The ambient temperature is the temperatureof the environment in which the device operates. According toembodiments to be described herein, the ambient temperature may beestimated without temperature sensors embedded in or directly attachedto the device surface.

FIG. 1 illustrates a simplified diagram of a portable device (“device100”) according to one embodiment. The device 100 includes a printedcircuit board (PCB) 110, which further includes one or more processors120 (e.g., central processing units (CPUs), graphic processing units(GPUs), digital signal processors (DSPs), and/or other types ofspecial/general-purpose processors or processing circuitry), one or morememory devices 130 (e.g., volatile memory such as random access memory(RAM), and/or non-volatile memory such as read-only memory (ROM), flashmemory, etc.), and other circuitry 140 such as memory controller,network adapter, clock circuitry, voltage regulator, etc. Additionalcomponents of the device 100 are not shown to simplify the description.

Before proceeding with the description of FIG. 1, a side view of thedevice 100 is shown according to one embodiment in FIG. 2. In the sideview shown in FIG. 2, the device 100 includes the PCB 110, which furtherincludes a system-on-a-chip (SoC) 280. In one embodiment, some or all ofthe processors 120 and the memory devices 130, as well as somecomponents of the other circuitry 140 may reside in the SoC 280. FIG. 1shows that a sensor 150 is mounted on the PCB 110 to sense or detect theinternal temperature of the device 100; e.g., the temperature of the PCB110. In an alternative embodiment, the sensor 150 may be attached to theSoC 280 (shown in FIG. 2), the processors 120 (shown in FIG. 1), orother circuit components. That is, the internal temperature may be thePCB 110 temperature, the SOC 280 temperature, or the temperature of anycircuit component to which the sensor 150 is attached. In oneembodiment, the device 100 may include more than one sensors 150 fordetecting the internal temperature.

Non-limiting examples of the device 100 may include: a cellphone, asmart watch, e-reader, or any handheld or wearable electronic devicewhose surface is in contact with the user's skin when in use.

Referring again to FIG. 1, the device 100 also includes a thermalcontroller 160, which performs dynamic thermal management on the device100 based on at least the estimation of the surface temperature.Although the thermal controller 160 is shown as a component outside thePCB 110, in alternative embodiments the thermal controller 160 may beimplemented on the PCB 110, or as part of an operating system (OS)stored in the memory 130 and containing an interface to hardware, tocontrol the voltage supply and/or clock circuitry to thereby dynamicallyadjust the voltage and/or frequency of the processors 120. Adjusting theprocessor voltage and/or frequency at runtime, in effect, adjusts theprocessors' power consumption and heat output. Referring to FIG. 1 andFIG. 2, in one embodiment, the device 100 may further include a powersupply 170 (e.g., battery), a display 180, and other hardwarecomponents.

The thermal controller 160 may include or be coupled to a temperatureestimator, an embodiment of which is shown in FIG. 1 as an estimator190. The estimator 190 estimates the surface temperature of the device100 using the internal temperature and the ambient temperature as inputto a circuit model, where the circuit model characterizes thermalbehaviors of the device 100. The parameters used in the circuit modelmay vary across different device product lines, and may be identified orchosen by the manufacturer of the device 100 based on results ofsimulations or experiments. The parameters are chosen such that theestimated surface temperature approximates the temperature of thewarmest part of the device surface, where the warmest part may be thedisplay, the back cover, the side surface, etc., depending on theapplications being executed and the hardware being utilized. Based onthe estimator's result, the thermal controller 160 may determine whetheror not to throttle the performance of the processors 120.

FIG. 3A and FIG. 3B illustrates the estimator 190 according to twoalternative embodiments. The estimator 190 may be implemented byhardware (e.g., general-purpose processors or Application-SpecificIntegrated Circuit (ASIC)), software (containing instructions stored inthe memory 130 and executable by the processors 120) or a combination ofboth. In FIG. 3A, the estimator 190 is a hardware component within orcoupled to the thermal controller 160. The estimator 190 receives thesensor 150 measurements of the internal temperature, and uses ambienttemperature (Tamb) parameters 310 and internal temperature (Tint)parameters 320 to calculate an estimated surface temperature of thedevice 100. The result of the estimation is used by the thermalcontroller 160 to control the operation of the processors 120.

In FIG. 3B, the estimator 190 is a software program stored in the memory130. The processors 120 executes the instructions of the estimator 190software program, using the sensor 150 measurements, Tamb parameters 310and Tint parameters 320 as input to estimate the surface temperature.The estimation performed by or according to the estimator 190 isdescribed below in connection with FIG. 4A and FIG. 4B.

FIG. 4A and FIG. 4B illustrate thermal-equivalent circuit diagrams forestimating the surface temperature according to some embodiments. Eachof these thermal-equivalent circuit diagrams shows a circuit model,which is an electric analog circuit describing the thermal behaviors ofthe device 100; e.g., heat conduction.

FIG. 4A shows a circuit model 400 including laddered RC circuitcomponents, where the RC circuit components include thermal capacitors(401, 402, 403) and thermal resistors (411, 412, 413, 414). Thesethermal capacitors and thermal resistors are the thermal equivalents oftheir electrical counterparts. The circuit model 400 is an electricaldescription of a thermal system, and it obeys the Kirchhoff's circuitlaws. The internal temperature (Tint) and the ambient temperature (Tamb)may be modeled as voltage sources 420 and 430, respectively. The surfacetemperature (Tsurf) may be modeled as an internal point 450 in thecircuit model 400. The thermal capacitors (401, 402, 403) and thethermal resistors (411, 412, 413, 414) model or describe the thermalbehaviors of the device 100, and are collectively referred to as theTint parameters 320. In alternative embodiments, the numbers of thermalcapacitors and thermal resistors, and/or the number of ladders in thecircuit model 400, may be different from the examples shown herein. Inthe circuit model 400, the temperatures at internal points N1, N2 andTsurf may be represented by the respective voltages at these points. Thecircuit model 400 can be used to calculate an estimated Tsurf, usingTint and Tamb as input, where Tint is measured by the sensor 150 ofFIG. 1. In one embodiment, Tamb may be a constant value, invariant oftime and device surroundings. Alternatively, Tamb may slowly change overtime (i.e., time-varying). A method of estimating Tamb will be describedwith reference to FIG. 5. The circuit model 400 describes thetemperature gradient from Tint to Tamb, in the direction of lowertemperature (i.e., from hot to cold).

FIG. 4B shows a circuit model 410 which includes the same thermalcapacitors (401, 402, 403) and thermal resistors (411, 412, 413, 414) asin the circuit model 400, but the voltage source 430 representing Tambis replaced by a ground terminal 440. The circuit model 410 can be usedwhen Tamb is a known constant value. The delta internal temperature(ΔTint) is the internal temperature value detected by the sensor 150minus Tamb (i.e., ΔTint=Tint−Tamb); in the circuit model 410, ΔTint ismodeled as a voltage output at the voltage source 420. The delta surfacetemperature (ΔTsurf) corresponding to ΔTint can be calculated usingΔTint and the Tint parameters 320 as input. More specifically, ΔTsurf isa temperature difference caused by heat transfer from the deviceinterior to the device surface. For example, the following threeequations may be formulated according to the Kirchhoff's circuit lawsapplied on the circuit model 410. The three unknown values (temperaturesat N1, N2 and ΔTsurf at point 450, all of which are represented byvoltages) can be solved from the following three equations.

${{\frac{V_{N1} - {\Delta T_{int}}}{R_{411}} + \frac{V_{N1} - V_{N2}}{R_{412}} + {C_{401}\frac{dV_{N1}}{dt}}} = 0}{{\frac{V_{N2} - V_{N1}}{R_{412}} + \frac{V_{N2} - {\Delta T_{surf}}}{R_{413}} + {C_{402}\frac{dV_{N2}}{dt}}} = 0}{{\frac{{\Delta T_{surf}} - V_{N2}}{R_{413}} + \frac{\Delta T_{surf}}{R_{414}} + {C_{403}\frac{d\left( {\Delta T_{surf}} \right)}{dt}}} = 0}$

After ΔTsurf is solved, Tsurf can be calculated by: Tsurf=ΔTsurf+Tamb.

FIG. 4C shows a circuit model 450 which includes two branches extendingfrom point N1, each branch with a voltage source representing arespective internal temperature detected by a respective sensor. Morespecifically, in this example two sensors are used to sense the internaltemperatures of the device. In alternative embodiments, more than twosensors may also be used. The surface temperature may be calculated bysolving a set of equations formulated according to the Kirchhoff'scircuit laws.

FIG. 5 is a flow diagram illustrating a method 500 of performing thermalmanagement based on the surface temperature of a portable device (e.g.,the device 100 of FIG. 1) according to one embodiment. The method 500may be performed by the device 100. The steps 510-540 may be repeated ata fixed time interval.

Referring also to FIG. 1, the method 500 begins with the estimator 190receiving the internal temperature of the device detected by the sensor150 (step 510). The estimator 190 determines the ambient temperature(step 520). In one embodiment, a constant value may be used as theambient temperature. In another embodiment, the ambient temperature isdetermined based on the state of the device 100, to be described withreference to FIG. 6. The surface temperature is then calculated usingthe internal temperature and the ambient temperature as input to acircuit model (step 530). The surface temperature may be used by thethermal controller 160 to determine whether to throttle the performanceof the device 100; e.g., by reducing the frequency of the processors120.

The ambient temperature may be estimated from a time-series of internaltemperatures; that is, the current internal temperature and thesuccessive past internal temperatures. Experiments have shown that theinternal temperature and its changes over time are correlated to theambient temperature when the device 100 is in the idle state (e.g., whenthe processors 120 are not in active operation). When the device 100 isnot idle (e.g., when at least one processor 120 is in the operatingstate), the internal temperature may fluctuate rapidly with processorusage, and therefore it may be difficult to discern a correlationbetween the internal temperature and the ambient temperature. Thus, inone embodiment, the ambient temperature is not updated when the device100 is in operation (i.e., not idle).

FIG. 6A is a flow diagram illustrating a method 600 for determining theambient temperature according to one embodiment. FIG. 6B is a flowdiagram illustrating a method 602 for determining the ambienttemperature according to another embodiment The methods 600 and 602include further details of the step 520 of FIG. 5 at which the ambienttemperature is determined based on the state of the device 100. Themethods 600 and 602 may be performed by the device 100 of FIG. 1.Although the methods 600 and 602 are described by referring to theinternal temperature sensed at one internal location of the device 100,it is understood that the methods 600 and 602 apply to internaltemperatures sensed at multiple locations of the device 100.

Referring also to FIG. 1, the method 600 begins with the estimator 190receiving (or obtaining) a state of the device 100 (e.g., the state ofthe processors 120) (step 610). If the state is idle (step 620), theambient temperature may be estimated using a time-series of internaltemperatures as input to an ambient temperature model (step 630). Theambient temperature model may be constructed prior to runtime of thedevice 100 based on calibrations of ambient temperatures vs. internaltemperatures, and may be stored in or accessed by the device 100. In oneembodiment, the ambient temperature model records the correlationbetween the ambient temperature and the internal temperatures over time.The estimation may be made by using the currently-detected internaltemperature and the successive past internal temperatures to determinethe correlated ambient temperature. In one embodiment, the internaltemperatures detected over time is a time-series of internaltemperatures which may be approximated by fitting an N-order polynomialto the time-series with N being any positive integer. In an alternativeembodiment, the ambient temperature model records the correlationbetween the ambient temperature and a corresponding internaltemperature, and the estimation may be made by using thecurrently-detected internal temperature to determine the correlatedambient temperature. If the state is not idle (step 620), the ambienttemperature is not updated; that is, the same ambient temperature as inthe previous time instant is maintained (step 640). Thus, if the device100 transitions from the idle state to the operating state, the ambienttemperature may be maintained at the same value as the most-recentlyestimated value in the idle state.

In FIG. 6B, the method 602 begins with the estimator 190 receiving (orobtaining) a state of the device 100 (e.g., the state of the processors120) (step 612). If the state is idle (step 622), the ambienttemperature may be estimated using a time-series of internaltemperatures as input to an ambient temperature model (step 632). Theambient temperature model has been described above with reference toFIG. 6B. If the state is not idle (step 622), the ambient temperaturemay be looked up from a table using the currently-detected internaltemperature, or a time-series of internal temperatures (step 642). Asmentioned before, the time-series of internal temperatures may beapproximated by fitting an N-order polynomial to the time-series with Nbeing any positive integer.

FIG. 6C is a flow diagram illustrating a method 650 for determining theambient temperature according to another embodiment. The method 650includes further details of the step 520 of FIG. 5 at which the ambienttemperature is determined based on the state of the device 100, andprovides an alternative to the steps 630 and 640 of FIG. 6A, as well asthe steps 632 and 642 of FIG. 6B. The method 650 may be performed by thedevice 100 of FIG. 1. Although the method 650 is described by referringto the internal temperature sensed at one internal location of thedevice 100, it is understood that the method 650 applies to internaltemperatures sensed at multiple locations of the device 100.

Referring also to FIG. 1, the method 650 begins with the estimator 190identifying a scenario in which the device 100 operates (e.g., theapplication runs on the processors 120) (step 660). The device 100 maystore or access a lookup table, which associates internal temperatureswith corresponding ambient temperatures based on calibrations performedprior to runtime of the device 100 for each of a set of scenarios inwhich the device 100 may operate. In one embodiment, one of thescenarios may be the idle state (i.e., no active operation). Once thescenario is identified, the ambient temperature may be looked up from atable using the currently-detected internal temperature, or atime-series of internal temperatures (step 670). As mentioned before,the time-series of internal temperatures may be approximated by fittingan N-order polynomial to the time-series with N being any positiveinteger.

FIG. 7 is a flow diagram illustrating a method 700 of estimating surfacetemperature of a portable device (e.g., the device 100 of FIG. 1)according to one embodiment. The method 700 begins with a sensordetecting an internal temperature of a portable device (step 710). Ascenario in which the portable device operates is identified (step 720).An ambient temperature of the portable device is determined using thescenario and at least the internal temperature (step 730). A surfacetemperature of the portable device is estimated using the internaltemperature and the ambient temperature as input to a circuit model thatdescribes thermal behaviors of the portable device (step 740).

The operations of the flow diagrams of FIGS. 5-7 have been describedwith reference to the exemplary embodiment of FIG. 1. However, it shouldbe understood that the operations of the flow diagrams of FIGS. 5-7 canbe performed by embodiments of the invention other than the embodimentdiscussed with reference to FIG. 1, and the embodiment discussed withreference to FIG. 1 can perform operations different than thosediscussed with reference to the flow diagrams. While the flow diagramsof FIGS. 5-7 show a particular order of operations performed by certainembodiments of the invention, it should be understood that such order isexemplary (e.g., alternative embodiments may perform the operations in adifferent order, combine certain operations, overlap certain operations,etc.).

Various functional components or blocks have been described herein. Aswill be appreciated by persons skilled in the art, the functional blockswill preferably be implemented through circuits (either dedicatedcircuits, or general purpose circuits, which operate under the controlof one or more processors and coded instructions), which will typicallycomprise transistors that are configured in such a way as to control theoperation of the circuitry in accordance with the functions andoperations described herein.

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention is notlimited to the embodiments described, and can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. The description is thus to be regarded as illustrative insteadof limiting.

What is claimed is:
 1. A method for estimating a surface temperature ofa portable device, comprising: detecting an internal temperature of theportable device; identifying a scenario in which the portable deviceoperates; determining an ambient temperature of the portable deviceusing the scenario and at least the internal temperature; and estimatingthe surface temperature using the internal temperature and the ambienttemperature as input to a circuit model that describes thermal behaviorsof the portable device.
 2. The method of claim 1, wherein detecting theinternal temperature further comprises: sensing a temperature of atleast one hardware component inside the portable device.
 3. The methodof claim 1, wherein the circuit model includes a plurality of thermalcapacitors, a plurality of thermal resistors, a first voltage sourcerepresenting the internal temperature, and a node representing thesurface temperature.
 4. The method of claim 3, further comprising:detecting one or more additional internal temperatures of the portabledevice, wherein in the circuit model the one or more additional internaltemperatures are represented by one or more additional voltage sourceslocated in different branches of a laddered RC circuit.
 5. The method ofclaim 4, wherein the circuit model further includes a second voltagesource representing the ambient temperature.
 6. The method of claim 4,wherein the circuit model further includes a ground terminalrepresenting the ambient temperature at a fixed value.
 7. The method ofclaim 4, wherein the surface temperature (Tsurf) is equal toTsurf=ΔTsurf+Tamb, where Tamb is the ambient temperature and ΔTsurf is atemperature difference caused by heat transfer from an interior to asurface of the portable device.
 8. The method of claim 1, furthercomprising: determining whether the portable device is in an idle statebefore determining the ambient temperature.
 9. The method of claim 8,wherein determining the ambient temperature further comprises:estimating the ambient temperature from a time-series of internaltemperatures when the portable device is in the idle state.
 10. Themethod of claim 8, further comprising: maintaining the ambienttemperature at a same value as a most-recently estimated value of theambient temperature in the idle state for use in the circuit model whenthe portable device is in an operating state.
 11. A portable deviceoperative to estimate a surface temperature of the portable device,comprising: a sensor to detect an internal temperature of the portabledevice; and circuitry to estimate the surface temperature, using theinternal temperature and an ambient temperature of the portable deviceas input to a circuit model that describes thermal behaviors of theportable device, wherein the circuitry is further operative to: identifya scenario in which the portable device operates; and determine theambient temperature using the scenario and at least the internaltemperature.
 12. The portable device of claim 11, wherein the sensor isoperative to sense a temperature of at least one hardware componentinside the portable device.
 13. The portable device of claim 11, whereinthe circuit model includes a plurality of thermal capacitors, aplurality of thermal resistors, a first voltage source representing theinternal temperature, and a node representing the surface temperature.14. The portable device of claim 13, wherein the sensor is operative tosense one or more additional internal temperatures of the portabledevice, wherein in the circuit model the one or more additional internaltemperatures are represented by one or more additional voltage sourceslocated in different branches of a laddered RC circuit.
 15. The portabledevice of claim 13, wherein the circuit model further includes a secondvoltage source representing the ambient temperature.
 16. The portabledevice of claim 13, wherein the circuit model further includes a groundterminal representing the ambient temperature at a fixed value.
 17. Theportable device of claim 13, wherein the surface temperature (Tsurf) isequal to Tsurf=ΔTsurf+Tamb, where Tamb is the ambient temperature andΔTsurf is a temperature difference caused by heat transfer from aninterior to a surface of the portable device.
 18. The portable device ofclaim 11, wherein the circuitry is further operative to determinewhether the portable device is in an idle state before determining theambient temperature.
 19. The portable device of claim 18, wherein thecircuitry is further operative to estimate the ambient temperature froma time-series of internal temperatures when the portable device is inthe idle state.
 20. The portable device of claim 18, wherein thecircuitry is further operative to maintain the ambient temperature at asame value as a most-recently estimated value of the ambient temperaturein the idle state for use in the circuit model when the portable deviceis in an operating state.